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I have a Sigma 50mm lens f1.4 and it has a lens hood that has a sine wave shape going around the perimeter of the lens.
I imagine this particular shape is useful for getting a particular effect and wonder what that is and how it would differ from just a straight edge.
The reason for using a hood is to block light rays from elements outside of the actual frame (stray light) to enter into the lens and degrade the image quality (lowering contrast). Effectively, it "shades" the lens from these rays. Since a sensor is rectangular, valid light rays that fall on the corners arrive from angles larger than light rays that fall on the middle of the sensor's edge. This is why many hoods have the "petal" form. Otherwise you will suffer from either vignetting at the frame corners, or lower effectiveness in blocking invalid rays.
With increasing focal length, the petal shape becomes less required and there is a transition to circular shape. Another reason for circular hoods is rotating front elements of the lens, where the hood is mounted. When focusing, the element (and hood) rotates so if you use petal hood it will lose its alignment with the sensor.
In addition to the already good posts, and likely forthcoming posts, I'll give one less common but equally important tip. Don't forget that using a lens hood will make it less likely that you'll cause damage to your lens, improving the quality with less dust and scratches on your lens.
Usually the lens hood is for stopping light from the sides to enter the lens and produce flare. It can also reduce illumination of dust or other dirt on the lens itself which manifests itself in the image.
The shape can be simply round and a tulip-shaped hood is usually only to prevent vignetting within the field of view.
